2. Field of the Invention
The present invention relates to an improved process for the preparation of 2,3-dihydro-2,2-dimethyl-7-hydroxybenzofuran from isobutenylpyrocatechol and/or methallylpyrocatechol.
2,3-Dihydro-2,2-dimethyl-7-hydroxybenzofuran (hereinafter, often simply "DDHB") is a compound having the structural formula: ##STR1##
Such compound is also designated 7-hydroxy-2,2-dimethylcoumaran.
DDBH is a known compound which is useful, in particular, as an intermediate in the synthesis of the insecticide carbofuran (2,3-dihydro-2,2-dimethylbenzofuranyl methylcarbamate).
Isobutenylpyrocatechol, as intended herein, is the compound 3-isobutenyl-1,2-dihydroxybenzene having the structural formula: ##STR2##
And methallylpyrocatechol, also as intended herein, is the compound 3-methallyl-1,2-dihydroxybenzene having the structural formula: ##STR3##
Methallylpyrocatechol is a compound which can be prepared by thermal isomerization of ortho-methallyl-oxyphenol; such isomerization is typically carried out by heating the parent compound at a temperature between 150.degree. and 250.degree. C., preferably in a medium diluted with a solvent.
Isobutenylpyrocatechol is a compound which can be prepared by prolonged heating, in bulk, of methallylpyrocatechol, preferably in contact with a metal surface, and with the subsequent elimination of any unreacted methallylpyrocatechol; such elimination can be carried out, e.g., chemically, the methallylpyrocatechol being converted into DDHB by heating (at a temperature between 100.degree. and 250.degree. C.) in the presence of water.
This invention especially relates to an economic process for the preparation of DDHB from isobutenylpyrocatechol, or admixtures thereof with methallylpyrocatechol.
2. Description of the Prior Art
The conversion of alkenylphenols into heterocyclic aryl compounds can be deemed either a cyclizing isomerization or an isomerizing cyclization.
The cyclizing isomerization of alkenylphenols has long been known to the art: in 1913, for example, German Pat. No. 279,864 described the conversion of allylphenol or allylcresol into methallylcoumaran in the presence of HBr or pyridine hydrochloride.
However, the preparation of compounds of the benzofuran type from compounds of alkenylphenol type is a much more complicated reaction than the simple cyclizing isomerization of an allylphenol, in particular, because of the risk that the isomerization might proceed in the direction of compounds other than the desired products.
In general, Claisen et al. [Ann., 442, page 228 (1925)] have concluded that the isomerization of simple allylphenol does indeed lead to compounds to benzofuran or coumaran type, in accordance with the equation: ##STR4## but that, on the other hand, the presence of substituents on the terminal carbon of the double bond leads to compounds of chroman type, in accordance with the equation: ##STR5## such being the case even if one of the radicals R represents hydrogen.
The immediately aforesaid is confirmed by more recent work, such as that of Frater et al. [Helv. Chim. Acta, 50, pages 255-256 (1967)] and Piccardi et al. [J. Chem. Soc., Perkin I, page 624 (1977)].
Certain methallylmonophenols too have been isomerized into benzofuran compounds by catalysis in an acid medium (isopropylmethallylphenol in U.S. Pat. Nos. 3,876,667 and 3,816,473 and acetylmethallylphenol in U.S. Pat. No. 3,419,579), but it is considered that, below 150.degree. C., the reaction proceeds too slow to be an acceptable process, with the result that the preferred temperatures are above 175.degree. C. (U.S. Pat. No. 3,419,579) or even above 200.degree. C. (U.S. Pat. Nos. 3,876,667 and 3,816,473).
In the particular case of monophenols substituted by nitro or bromo groups, it is reported (British Pat. No. 1,139,001 and French Pat. No. 1,472,283) that the corresponding methallylphenols and isobutenylphenols can be cyclized/isomerized in the presence of catalysts, at above 100.degree. C. However, temperatures above 150.degree. C. are still preferred and the foregoing patents do not allude to the cyclizing isomerization of isobutenylphenols, but only of nitro- or bromo-methallylphenol, in the presence of a catalyst of HBr/acetic acid type.
The isomerization of methallylphenol is obviously a simpler reaction than the isomerization of isobutenylphenol because, as Claisen et al. had already reported, the substituents on the terminal carbon of the double bond tend to direct the cyclizing isomerization in the direction of compounds other than benzofuran compounds.
Furthermore, the isomerization of isobutenylphenols is also a much more difficult reaction because of the fact that the double bond in the isobutenyl substituent is in a conjugated position relative to the aromatic nucleus; this chemical configuration, resembling that existing in the case of styrene, favors polymerization reactions at the expense of isomerization.
However, the cyclizing isomerization of isobutenylphenol has been expressly described by Martini et al. [J. Org. Chem., 35(9), 2,904-2,906 (1970)], who report that, in order to obtain a suitable yield (84%), it is necessary to heat the isobutenylphenol in question (5-methyl-2-isobutenylphenol) in the presence of MgCl.sub.2 for 8.5 hours at 184.degree.-194.degree. C.
Thus, all of the prior art would appear to be in agreement that:
(i) The isomerization of alkenylphenols substituted on the terminal carbon of the double bond is a difficult reaction;
(ii) Certain very special substituents can facilitate this isomerization, but only nitro and bromo groups are known to exert such influence;
(iii) Only relatively harsh and special conditions of temperature and reaction time can normally permit the conversion of an isobutenylphenol into a benzofuran; and
(iv) The cyclizing isomerization of isobutenylphenols is a much more difficult reaction than the isomerization of allylphenols.
Nonetheless, all of the aforesaid prior art in fact relates only to alkenylmonophenols. The isomerization of alkenylpyrocatechol has scarcely been mentioned. In French Pat. No. 1,430,952, only the cyclization of allylpyrocatechol is reported, but no mention is made as regards the cyclization of isobutenylpyrocatechol. Furthermore, U.S. Pat. Nos. 3,816,473 and 3,876,667 advise against the use of pyrocatechol (designated simply as "catechol") in the synthesis of carbofuran intermediates, because of the presence of the two reactive hydroxyl groups and because of the tendency of pyrocatechol to give rise, on the one hand, to large amounts of undesirable by-products, and, on the other hand, to desired products of but mediocre purity.